58 
SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
age, which is given in the third column of the table, may be regarded as typical of 
this interesting and apparently unique type of rooting slate. 
Analyses of igneous green slate, Eureka quarry, Slatington, Cal. 
Aver- 
age. 
Silica (Si0 2 ) 
Alumina ( A1 2 3 ) 
Iron oxides (FeO, F 2 
Lime (CaO) 
Magnesia (MgO) 
Sulphur (S) 
Alkalies (K 2 0,Na 2 0) . 
Carbon dioxide (C0 2 ) 
Water 
46.22 
15.93 
8.21 
7.12 
8.29 
.12 
3.17 
10.60 
a Not determined. 
These analyses have been compared a with a series, of 36 analyses of American 
roofing slates derived from clays by pressure, and remarkable differences in composi- 
tion are apparent. The green slate, on the other hand, approximates closely in 
composition to certain "basic" igneous rocks of the district, and it is probable that 
it was derived from a gabbro or similar rock. 
The "green slate" is in reality grayish green in color. It splits readily, though 
with not so smooth a surface as the black slate. It stands punching and trimming 
well, and is sufficiently strong for roofing use. Considering its origin and composi- 
tion it is probable that it will be a highly durable slate, holding its color well. At 
present it is sold entirely for trimming and lettering on black slate roofs, for which 
purpose it is particularly well adapted, giving a strong but pleasant color contrast. 
Chili Bar Slate Company quarry. — This quarry is located about 3 miles north of 
Placerville, in sec. 36, T. 11 N., R. 10 E., on the south side of the South Fork of 
American River, a few hundred yards east of the Placerville-Kelsey stage road. 
This is the oldest quarry in the district, having been opened about twenty years 
ago. It has been shut down since 1897. 
Several openings were made in a bluff forming the river bank at this point. In 
the easternmost of these openings, which is about 40 feet high and 30 feet wide, a 
rather poor slate with irregular joints is shown. The cleavage strikes N. 20° W., 
and dips 75° E. The westernmost opening is small, with a tunnel which was appar- 
ently run in on a band of better slate. The slate piled in the yard has kept its color 
fairly well. 
It seems possible that this quarry may be flooded at high water. Both it and the 
one next mentioned (San Francisco quarry) are badly located, having no large dump- 
ing area available near the quarries. Neither quarry has gone deep enough to get 
really good slate, which might have been found at a greater depth. 
San Francisco Slate Company quarry. — This quarry is located in T. 11 N., R. 10 E., 
within a quarter mile of that of the Chili Bar Slate Company, but on the north side 
of the river and west of the Placerville-Kelsey road. The principal opening was 
located about 600 feet north of the river, at an elevation of 150 feet above its bank. 
A tramway led down to the dressing yards, which were situated at the river bank. 
The cleavage of the slates in the large opening strikes about N. 30° W., and has 
an almost vertical dip. No slate has been quarried here since 1897. A large stock 
of trimmed slates is still piled in the dressing yard, and many of these have already 
discolored badly. 
« Jour. Geol., vol. 12, 1904, p. 26. 
